Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
Background Art
In conventional power semiconductor devices, vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) that have a trench structure are manufactured to reduce the on-resistance of the device. In vertical MOSFETs, a trench structure in which a channel is formed orthogonal to the substrate surface makes it possible to achieve a higher cell density per unit area than a planar structure in which a channel is formed parallel to the substrate surface. This makes it possible to increase the current density per unit area and is advantageous from a cost perspective.
However, when a trench structure is formed in a vertical MOSFET, the inner walls of the trench structure are entirely covered by a gate insulating film so that the channel can be formed in the vertical direction, and the portion of the gate insulating film at the bottom of the trench is positioned near the drain electrode. As a result, the portion of the gate insulating film at the bottom of the trench tends to be subjected to strong electric fields. Particularly in the ultra-high breakdown voltage devices manufactured using wide-bandgap semiconductors (semiconductors that have a wider bandgap than silicon, such as silicon carbide (SiC)), the negative effects on the gate insulating film at the bottom of the trench can significantly decrease the reliability of a vertical MOSFET.
One proposed solution to this problem is a structure in which a p-type region that contacts a p-type base region is formed extending down to a position deeper than the bottom of the trench and the p-n junction is formed at a position deeper than the bottom of the trench in order to decrease the magnitude of the electric field at the bottom of the trench (see Patent Document 1, for example). Structures in which a p-type region is formed at the bottom of the trench have also been proposed (see Patent Document 2, for example). Still another proposed solution is a structure that combines both the structure in which a p-type region that contacts a p-type base region is formed extending down to a position deeper than the bottom of the trench and the p-n junction is formed at a position deeper than the bottom of the trench and the structure in which a p-type region is formed at the bottom of the trench (see Patent Document 3, for example).